Light sources for projection lamps have generally comprised incandescent lamps, the source of light being a coiled tungsten filament heated to incandescence by electric current flow therethrough. It is known to use interference filters in such lamps. U.S. Pat. Nos. 3,662,208 and 4,017,758 disclose the use of infrared-reflecting interference filters in front of reflector lamps to prevent undue heating of objects illuminated by the lamps. U.S. Pat. Nos. 4,095,881 and 3,209,188 disclose the use of such filters with tungsten-halogen lamps to reflect heat back to the lamp in order to aid in maintaining the lamp wall above the temperature necessary for the halogen cycle. U.S. Pat. Nos. 3,704,928 and 3,528,725 disclose the use of interference filters to correct the color of emitted light. The prior art does not disclose the use of interference filters with projection lamps to improve lamp efficacy (lumens per watt). Although it is known that the efficacy of an incandescent lamp can be improved by means of a suitable interference filter on the lamp envelope (see U.S. Pat. No. 4,160,929 and Offenlegungsschrift No. 24 48 348, which corresponds to U.S. application Ser. No. 410,914, filed Oct. 23, 1973), it is difficult to actually improve efficacy, unless the filament is relatively large, because of the problem of accurately positioning the filament at the axis of the envelope.
In reflector lamps having interference filters in which the light source is an arc discharge lamp, the interference filter generally reflects visible light and transmits infrared radiation in order to prevent undue heating of the illuminated object, as shown in U.S. Pat. Nos. 3,174,067 and 3,379,868, or in order to avoid high temperatures on external lamp surfaces, as in U.S. Pat. No. 4,197,480.